This invention relates to permanent rotor brushless d-c motors which are energized from a d-c source using commutation transistors controlled by means sensing the angular position of the rotor in general, and more particularly to an improved circuit arrangement for use with such a motor which permits using unmatched commutation transistors.
It is well known that due to manufacturing tolerances, transistors of a given type will have very different current gain from one transistor to the other. When such transistors are used in a brushless d-c motor of the type described above, different currents can be caused to flow in the different stator windings. As a result, the torque of the motor will fluctuate. In order to avoid such variations in motor torque, it is necessary to use commutation transistors which have, to the greatest degree possible, equal current gains. Typically, such has been done by selecting transistors from a large number to obtain transistors which are matched in current gain. Obviously, this method requires a great deal of time and is quite expensive, thereby greatly increasing the manufacturing cost of such motors. Those skilled in the art will recognize that the cost of matched sets of transistors greatly exceeds the cost of the same transistor by itself.
Thus, it can be seen that there is a need for a brushless motor in which unmatched transistors can be used, thereby avoiding the process of transistor selection or the higher cost of buying prematched transistors, and which motor using unmatched transistors is still able to operate without the undesirable variations in motor torque.